Article pubs.acs.org/Macromolecules
Ion Clustering in Quaternary Ammonium Functionalized Benzylmethyl Containing Poly(arylene ether ketone)s Dongyang Chen and Michael A. Hickner* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Quaternary ammonium functionalized poly(arylene ether ketone)s (QA-PAEKs) with mass-based ion exchange capacities (IECg) ranging from 1.12 to 2.88 mequiv g−1 were synthesized via condensation polymerization of a newly designed highly benzylmethylated bisphenol, subsequent bromination of the benzylmethyl groups, and then quaternization with trimethylamine. The quaternary ammonium groups were densely and selectively anchored on the bis(3,5-dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane residues in the QA-PAEK backbone. 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to confirm the chemical structure of the samples. The morphology, mechanical properties, thermal stabilities, water uptake, swelling ratio, and bromide (Br−) conductivity of the QA-PAEK membranes were investigated. It was found that QA-PAEK samples had much higher Br− conductivity than a randomly functionalized quaternary ammonium Radel (QA-Radel) membrane at similar IEC, which was attributed to the existence of distinct ion clusters in the QA-PAEK materials as evidenced by small-angle X-ray scattering (SAXS). The Br− conductivity of the QA-PAEK membranes increased with increasing quaternary ammonium ion concentration, water uptake, temperature, and conducting volume fraction. The results indicated that ion clustering was important for enhanced ion conductivity, and QA-PAEKs are promising anion exchange membranes for use in energy and water treatment applications.
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INTRODUCTION Quaternary ammonium functionalized polymers are wellknown cationic polyelectrolytes that have found numerous successful applications in the chemical, medical, and biological fields.1,2 Membrane-based processes for water-insoluble cationic polymers include fuel cells,3,4 flow batteries,5 water electrolyzers,6,7 electrodialysis,8 and reverse electrodialysis.9 In these membranes, positively charged quaternary ammonium groups are tethered to the polymeric backbones while the negatively charged counterions become mobile upon solvation. Therefore, these materials are termed anion exchange membranes (AEMs) where the anion motion can be used for the transport of charge or the fixed cationic groups can be employed for exclusion of mobile cations. Currently, knowledge concerning the structure−morphology−performance relationships of AEMs is much less developed compared to that of sulfonated proton exchange membranes (PEMs) with fixed anionic charges on the polymer backbone.10 Many types of PEMs have been studied in depth over the past decade, primarily for fuel cell applications, and insights into their material properties, ionic domain structures, and their interaction with water molecules have been widely reported.11,12 However, the most promising strategies for producing high-performance AEMs are not yet clear. © 2013 American Chemical Society
Ion clustering is a key factor for enhanced ion conductivity in PEMs. The importance of the existence of the ionic domain morphology is a well-established finding from Nafion membranes and has been validated by work on several aromatic sulfonated polymers that display distinct ion clustering, or hydrophilic−hydrophobic phase separation, as observed by scattering and microscopy technuques.13,14 The hydrophilic acidic groups in PEMs tend to segregate from the hydrophobic polymer backbones during the membrane casting process. The phase connectivity and domain size can be modulated by the polymer backbone composition and chain topology.12,13 To facilitate the formation of ionic domains, measures can be taken to place the acid groups on polymer grafts or at the ends of chains15 or clustered in certain segments of the polymer chains16−19 including densely functionalized ionic monomers or block copolymers. It has been found in many cases that the ion conductivity of PEMs with obvious ion clustering is much higher than that of the PEMs without ion clustering, owing to the fact that ion clustering contributes to the formation of the hydrophilic channels for the fast diffusion of water and Received: August 1, 2013 Revised: November 11, 2013 Published: November 22, 2013 9270
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conduction of ions through the membrane.12 Inspired by these findings in PEMs, quaternary ammonium functionalized block copolymers have been synthesized and enhanced ion conductivity has been observed.20−26 However, only a few such systems have been reported so far. Block copolymers are a promising model system, but other easy-to-access methods of creating phase separation in AEMs are desirable.27 Many existing AEM studies are focused on improving the alkaline stability of cationic membranes by screening different cation groups such as imidazolium, 28 phosphonium, 29 guanidinium,30 sulfonium,31 and pyridinium moieties.32 The alkaline stability of AEMs is particularly important for alkaline fuel cells where high pH, low relative humidity, and elevated temperature are the target operating conditions.20 Many useful concepts have been demonstrated in this space, but currently no widespread breakthrough for stabilizing AEMs against degradation has been established over the baseline of simple alkyltrimethyl and benzyltrimethyl quaternary ammonium groups.33−35 Therefore, most device demonstrations at low pH conditions employ materials with quaternary ammonium functionalization because of their ease of synthesis and reasonable performance. A challenging issue for the currently available AEMs is their low anion conductivities, primarily because of the low mobility of anions as compared to the mobility of protons in PEMs.36 Therefore, manipulating the morphology of AEMs to increase their anion conductivity is extremely important for next-generation materials. The synthesis of quaternary ammonium functionalized AEMs generally involves halomethylation of a polymer and subsequent reaction with the desired amine precursors.37−39 For the halomethylation, chloromethylmethyl ether is the most popular reagent, which is highly carcinogenic.39 Furthermore, only one site on a benzene ring is functionalizable by chloromethylmethyl ether before side reactions and cross-linking occur. Alternatively, N-bromosuccinimide is a much safer halomethylation reagent than chloromethylmethyl ether and can impart multiple functional groups onto a methylated benzene ring depending on the number of benzylmethyl substituents.40,41 In order to develop high performance AEMs with superior morphology for anion conductivity from low-risk, scalable synthetic steps, herein we report the synthesis of quaternary ammonium functionalized benzylmethyl containing poly(arylene ether ketone)s (QA-PAEKs) using N-bromosuccinimide as halomethylation reagent. The quaternary ammonium groups were densely anchored at the predesigned benzyl positions on the monomer units for efficient phase separation. Detailed physicochemical properties of QA-PAEKs were measured and correlated to the ion conducting properties of the membranes.
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powder was obtained as crude product, which was washed with deionized water and then recrystallized from toluene−ethanol (volume ratio 3:1) solutions twice. A new bisphenol monomer, bis(3,5dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane, was obtained with ∼40% yield. The reaction route is depicted in Scheme 1.
Scheme 1. Synthesis of Bis(3,5-dimethyl-4-hydroxyphenyl)3,5-dimethylphenylmethane
Polymer Synthesis. The synthesis and subsequent functionalization of the polymers in this work are depicted in Scheme 2. First, the above obtained monomer, bis(3,5-dimethyl-4-hydroxyphenyl)-3,5dimethylphenylmethane, was polymerized with bisphenol A and 4,4′-difluorobenzophenone to produce benzylmethyl containing poly(arylene ether ketone)s (PAEKs) with 10, 20, 30, 40, and 50 mol % of the total bisphenol loading being bis(3,5-dimethyl-4-hydroxyphenyl)3,5-dimethylphenylmethane. Target stoichiometies of bisphenol to activated dihalide were 1:1 mol:mol to ensure high molecular weight step-growth copolymerization. A typical polymerization procedure follows: to a 100 mL three necked round-bottom flask equipped with a Dean−Stark trap, an argon inlet and outlet, a condenser, a magnetic stirrer, 2.0850 g (15.0 mmol) of K2CO3, 30 mL of DMSO, 15 mL of toluene, 1.8025 g (5.0 mmol) of bis(3,5-dimethyl-4-hydroxyphenyl)3,5-dimethylphenylmethane, 1.1415 g (5.0 mmol) of bisphenol A, and 2.1820 g (10.0 mmol) of 4,4′-difluorobenzophenone were charged. The reaction was carried out at 145−150 °C for 3 h, and then the toluene was distilled from the reaction mixture with the produced water. The temperature was raised to 170 °C and maintained for 15 h. Afterward, the viscous mixture was poured into water to form the fibrous product PAEK-50, which was collected by filtration. The number in the sample name represents the mole percentage of bis(3,5dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane to the total bisphenol monomer content in the synthesis of the polymer. All the synthesized PAEKs were obtained with ∼99% yield. The PAEK samples were brominated using N-bromosuccinimide (NBS) as the bromination agent. A typical procedure was given as follows: to a 250 mL flask, 2.4 g (5.0 mmol) of PAEK-50, 100 mL of dichloroethane, 2.7 g of N-bromosuccinimide (NBS) (15.0 mmol), and 0.2 g of benzoyl peroxide (BPO) (0.8 mmol) were added. The reaction was carried out at 80 °C for 5 h under argon. Upon cooling to room temperature, the reaction mixture was poured into 500 mL of methanol. Fibrous product was formed and collected by filtration. The product (Br-PAEK-50) was washed with 100 mL of acetone three times, dissolved in chloroform, precipitated in methanol, collected by filtration, and dried under vacuum at 40 °C for 24 h. The prefix “Br” in the sample name designates brominated precursor polymer before quaternization. After bromination, the polymers were quaternized by trimethylamine in N,N′-dimethylacetamide (DMAc) solution using the Menshutkin reaction at room temperature.33 In a typical procedure, 3.2 g (5 mmol) of brominated Br-PAEK-50 was dissolved in 25 mL of DMAc in a glass vial. 1.8 g of trimethylamine solution (45 wt % in water) was added, and the container was sealed. The mixture was magnetically stirred at room temperature for 24 h, then poured into a Petri dish, and dried at 70 °C under ambient pressure for 24 h. A solid product (QA-PAEK-50) was obtained. The prefix “QA” designates “quaternary ammonium” functionalized polymer. Membrane Preparation. QA-PAEK samples were redissolved in DMAc as ∼8 wt % solutions, cast onto leveled glass plates, and dried at 70 °C under ambient pressure for 24 h followed by vacuum drying at 100 °C for an additional 24 h. The dried membranes were peeled from the casting substrate and immersed in deionized water for 24 h to
EXPERIMENTAL METHODS
Materials. Quaternary ammonium functionalized Radel (QARadel) with an ion exchange capacity (IEC) of 2.36 mequiv g−1 was synthesized according to a previous report.42 All other chemicals were purchased from common commercial suppliers and used as received. Bis(3,5-dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane Monomer Synthesis. 13.4 g (0.1 mol) of 3,5-dimethylbenzaldehyde and 36.7 g (0.3 mol) of 2,6-dimethylphenol were dissolved in 200 mL of toluene in a 250 mL three-neck round-bottom flask with magnetic stirring at room temperature (25 °C) under the protection of argon. 5 mL of sulfuric acid (98 wt %) was added dropwise to the solution, and then the reaction temperature was increased to 70 °C and held for 24 h. Afterward, the reaction was cooled to room temperature, and the precipitate was collected by filtration. A yellowish 9271
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Scheme 2. Synthesis of PAEKs, Br-PAEKs, and QA-PAEKs
remove any remaining DMAc residue. The membranes were then stored in fresh deionzied water until use. Characterization. 1H nuclear magnetic resonance (NMR) and 13C NMR spectra were recorded on a Bruker DRX-400 NMR instrument and the chemical shifts were listed in parts per million (ppm) downfield from tetramethylsilane (TMS). FTIR spectra were recorded on a Bruker Vertex 70 FTIR spectrometer in transmission. The thermal stability of the QA-PAEK samples was analyzed using a Q50 TGA (TA Instruments Corp.). The temperature was increased from room temperature to 100 °C and held for 30 min to remove the absorbed water from the sample and then increased to 650 °C at a heating rate of 10 °C min−1 under a N2 atmosphere. Gel permeation chromatography (GPC) analysis was performed on a Waters Breeze system equipped with a Waters Styragel column, Waters 515 HPLC pump, and Waters 2414 refractive index detector. Chloroform was used as the elution solvent with the flow rate of 1 mL min−1. Nearly monodisperse polystyrene standards were used for GPC calibration. The mechanical properties of the wet membranes were measured on an Instron 5866 universal testing machine at room temperature. Smallangle X-ray scattering (SAXS) patterns were collected on a Rigaku (formerly Molecular Metrology) instrument with a pinhole camera with Osmic microfocus source and parallel beam optic. The instrument was equipped with a Cu target (λ = 1.542 Å) and a multiwire area detector. Measurements were carried out at room temperature under vacuum. X-ray photoelectron spectroscopy (XPS) was recorded on Kratos Analytical Axis Ultra XPS instrument with charge corrected to C at 285 eV. The mass-based ion exchange capacities (IECg) of the membranes were calculated from the 1H NMR spectra and confirmed by titration. For the 1H NMR method, the mole percent of the quaternary ammonium groups was calculated from the integral ratio of the methylene peak to the polymer backbone peaks. For titration, 0.2 g of membrane was immersed in 50 mL of 0.2 M NaNO3 solution for 24 h
and titrated with 0.1 M AgNO3 using K2CrO4 as a colorimetric indicator. The IECg was calculated from IECg =
ΔVAgNO C AgNO 3
3
md
(1)
where md is the mass of the dry membrane, ΔVAgNO3 is the consumed volume of AgNO3 solution, and CAgNO3 is the concentration of AgNO3 solution. The water uptake (WU) of the membranes was defined as weight ratio of the absorbed water to that of the dry membrane as given by
⎛ m − md ⎞ WU = ⎜ w ⎟ × 100% ⎝ md ⎠
(2)
where md and mw are the weight of membranes before and after water absorption, respectively. The hydration numbers (λ) were calculated from
⎞ ⎛ m − md ⎞⎛ 1000 ⎟⎟ λ=⎜ w ⎟⎜⎜ ⎝ md ⎠⎝ M H2O × IEC ⎠
(3)
where MH2O is the molecular weight of water. The water uptake at different relative humidities (RHs) at 30 °C was measured by a TA Instruments Q5000SA water vapor sorption microbalance. The concentration (c) of quaternary ammonium ions in the wet membranes was calculated from the titrated IECg and water uptake via the equation
c=
IECg 1 ρpolymer
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+
WU 100ρwater
(4)
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where ρpolymer is the density of the membranes (membrane mass divided by membrane volume) and ρwater is the density of water (1 g cm−3). The ion conductivities of the membranes were measured by twoprobe electrochemical impedance spectroscopy (EIS) using a Solartron 1260A frequency response analyzer as previously reported.43 Fully hydrated conductivity measurements were carried out in deionized water. A temperature and relative humidity controlled chamber (Espec SH-241) was employed for RH conductivity measurements. The Br− conductivity (σBr−) was calculated from the impedance plot according to the cross-sectional area available for ion transport (A) and the distance between electrodes (d), σ = d/RA, where the resistance, R, was derived from the intercept of the low frequency complex impedance with the Re(Z′) axis. The Arrhenius activation energy (Ea) for ion conduction was calculated from the slope of ln σBr− versus 1/T through
ln σBr− = ln σ0 −
Ea RT
confirmed its chemical structure. The benzylmethyl groups in this monomer were designed as highly clustered reaction sites for quaternary ammonium functionalization. Condensation polymerization of this benzylmethyl-containing monomer with bisphenol A and 4,4′-difluorobenzophenone was carried out using nucleophilic aromatic step-growth copolymerization,33,44 and high molecular weight benzylmethyl containing polymers (PAEKs) were obtained as shown in Table 1. Subsequent bromination of the polymer was conducted successfully at the benzylmethyl positions using NBS, as evidenced by the new 1H NMR peak at 4.25−4.51 ppm in Figure 2b compared to the 1H NMR of the unfunctionalized
(5)
where σBr− is the ion conductivity, σ0 is the pre-exponential factor, R is the gas constant, and T is the absolute temperature.
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RESULTS AND DISCUSSION Monomer and Polymer Synthesis. A new multibenzylmethyl containing bisphenol monomer, bis(3,5-dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane, was synthesized successfully from 3,5-dimethylbenzaldehyde and 2,6dimethylphenol. The 1H and 13C NMR spectra in Figure 1
Figure 2. 1H NMR spectra of (a) PAEK-20, (b) Br-PAEK-20, and (c) QA-PAEK-20.
PAEK in Figure 2a. Around 70 mol % of the benzylmethyl groups were brominated under our reaction conditions based on the 1H NMR results. While we did find that increasing the loading of NBS and BPO or increasing the reaction temperature could lead to higher degrees of bromination, reduced molecular weight polymer was obtained.40 Therefore, we did not pursue materials with higher degrees of bromination in this work. In Table 1, the molecular weight of PAEKs did not change significantly after bromination, while the polydispersity indices (PDI) were slightly higher for the brominated samples compared to the starting material. Treating Br-PAEK samples with trimethylamine converted the bromomethyl groups to quaternary ammonium groups in quantitative yield, as
Figure 1. 1H NMR (upper) and 13C NMR (lower) spectra of bis(3,5dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane.
Table 1. GPC Results for the PAEK Samples before and after Bromination before bromination −1
−1
after bromination
sample
Mn (kg mol )
Mw (kg mol )
PDI
Mn (kg mol )
Mw (kg mol−1)
PDI
PAEK-10 PAEK-20 PAEK-30 PAEK-40 PAEK-50
34.4 34.7 34.4 51.3 45.2
65.4 74.0 62.5 98.8 95.0
1.9 2.4 1.9 1.9 2.1
32.3 36.1 35.7 31.3 41.0
63.6 94.3 79.0 65.7 94.3
2.1 2.6 2.3 2.1 2.3
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Table 2. Properties of QA-PAEK Copolymers sample
density (g cm−3)
IECg from 1H NMR (mequiv g−1)
IECg from titration (mequiv g−1)
c (mmol cm−3)
water uptake (wt %)
swelling ratio (%)
Ea (kJ mol−1)
QA-PAEK-10 QA-PAEK-20 QA-PAEK-30 QA-PAEK-40 QA-PAEK-50
1.01 1.14 1.20 1.26 1.32
1.13 1.80 2.33 2.65 2.92
1.12 1.79 2.23 2.49 2.88
1.01 1.75 2.11 2.31 2.46
8 14 23 29 41
3 8 13 20 33
27 26 26 26 26
evidenced by the 1H NMR spectrum in Figure 2c. QA-PAEK materials were obtained with this simple final step. To give more chemical structure information on the QAPAEK samples, the FTIR spectrum of QA-PAEK-30 was measured, as shown in Figure S1 (Supporting Information). The absorption peaks of the constituent benzene groups, methyl groups, and ketone groups were all indexed in the spectrum; however, the absorption peaks for the C−N bond in the quaternary ammonium groups were not discernible, similar to other reported quaternary ammonium containing polymer systems.24,29 This absence of the C−N vibrational signature in the QA moieties may be attributed to the low absorption intensity of the C−N bond and the overlap of C−N peaks with the absorption peaks of other functional groups in the polymer. In order to detect the presence of the quaternary ammonium groups in a straightforward way, XPS was employed. A clear N 1s signal at 399.8 eV was observed in the XPS spectrum (Figure S2). The peaks for the counteranion, Br−, were also detected at 255.8, 181.8, and 68.8 eV, which corresponded to the Br 3s, 3p, and 3d emissions, respectively. All these data supported the successful synthesis of the target QA-PAEK samples with backbone-tethered quaternary ammonium groups. The IECgs for QA-PAEKs are listed in Table 2. The quaternary ammonium groups had methyl and methylene protons in the 1H NMR spectra. The integral ratio of these protons to the aromatic protons from the polymer backbone was used to calculate the IECgs of the samples. It can be seen that the IECg increased monotonically with an increasing amount of bis(3,5-dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane monomer residue in the polymers, as expected. The titrated IECgs agreed well the IECgs from 1H NMR (Table 2). The concentration of ions, c in Table 2, is another important parameter for the hydrated ionomers.45 It can be seen that c increased with an increasing amount of bis(3,5dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane used in the synthesis of the polymer. The rate of increase in c was decreased as the IECg grew larger, suggesting that QA-PAEKs had not yet reached but were approaching their highest ion concentration before significant swelling set in an ion dilution occurred. Membrane Morphology. The quaternary ammonium groups in QA-PAEK copolymers were anchored densely on the bis(3,5-dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane monomer residues to facilitate the formation of ion clusters. To validate this membrane morphology hypothesis according to the polymer chemical structure, SAXS was employed, and the results are shown in Figure 3. A QARadel membrane with quaternary ammonium groups dispersed randomly along the poly(arylene ether sulfone) backbone from chloromethylation functionalization was chosen for comparison. The SAXS pattern of the QA-Radel sample did not show an ionomer peak, indicating that no ionic clustering was present in the dry sample. For QA-PAEK membranes, obvious ionomer
Figure 3. SAXS patterns of QA-PAEK samples and QA-Radel.
or length scale correlation peaks were observed, and their intensities decreased with an increase in IECg. The interdomain spacing distances (d) were found to be 8.2, 7.4, 6.4, 5.3, and 4.9 nm for QA-PAEK-10, QA-PAEK-20, QA-PAEK-30, QA-PAEK40, and QA-PAEK-50, respectively. This result demonstrated that there were ion clusters in QA-PAEKs membranes, which originated from the densely anchored quaternary ammonium groups on a specific segment of the polymer backbone. The ion clusters were more organized in low IECg QA-PAEK samples than in high IECg QA-PAEK materials because of increased phase mixing and shorter nonionic segment length in high IECg QA-PAEKs. Membrane Properties under Fully Hydrated Conditions. The room temperature water uptake of QA-PAEK samples increased with increasing IECg, ranging from 8 to 41 wt %, as shown in Table 2. Water is critical for ion pair separation and ion conductivity in AEMs; however, higher water uptake led to larger swelling ratios as shown in Table 2. All of the reported membranes were flexible and mechanically strong. The Young’s moduli of the QA-PAEK samples were considerably higher than that of the N212 membrane (Table S1). The yield strain and yield stress of QA-PAEK samples decreased with increasing water uptake because of the plasticization by water. Except for QA-PAEK-50, all of the QA-PAEK samples had higher yield stresses than N212. The elongation at break values for all aromatic-based QA-PAEK samples were lower than for N212. Elongation at break is a critical value for membranes as it is a measure of their resistance to cracking under cyclic stress. The values for the QA-PAEK samples were acceptable, and all membranes were not brittle in our characterization measurements. For anion conductivities, the Br− conductivity is generally lower than the OH− and Cl− conductivities because of the low dilute solution mobility of Br− and less water uptake in halide form samples compared to bicarbonate and hydroxide form samples.46 However, the conducting ion species in AEMs varies across different applications so it is valuable to assess the anion 9274
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conductivity of AEMs with a number of different mobile ions. We have observed previously that poly(arylene ether) backbones are susceptible to degradation under basic conditions.47 Therefore, only Br− conductivity was reported herein for fundamental AEM studies. The Br− conductivities of QAPAEKs at room temperature are shown in Figure 4 along with
Figure 5. Br− conductivity of QA-PAEK samples as a function of temperature.
promising in comparison to block anion exchange membranes, given the fact that the diffusion coefficient of Br− (1.85 × 10−9 m2 s−1) is similar to the diffusion coefficient of Cl− (1.77 × 10−9 m2 s−1) at infinite dilution in water at 25 °C.50 Membrane Properties under Partially Hydrated Conditions. Partially hydrated conditions are extremely important for investigating AEMs because in many cases the AEMs are hydrated only through water vapor carried into the cell, such as in gas-fed fuel cells. Also, surveying the range of properties of AEMs as a function of water content provides insights into how the hydration of the sample affects the ion conductivity and morphology changes during the absorption of water.51 The water uptake of the QA-PAEK samples as a function of relative humidity (RH) is shown in Figure 6. The
Figure 4. Br− conductivity of QA-PAEK samples and QA-Radel at room temperature in liquid water.
the conductivity of QA-Radel, for comparison. The Br− conductivity of the QA-PAEK samples increased gradually with increasing IEC, while the Br− conductivity of the QARadel sample was below the trend line for the QA-PAEK materials. This observation was in good accordance with the morphology results from SAXS. QA-Radel is a randomly functionalized polymer that displays no evidence of nanophase structure in SAXS. In this case for QA-Radel, the ionic groups do not form obvious ion-rich domains to constitute the hydrophilic channels for efficient Br− conduction, similar to the case in some PEMs.48 However, QA-PAEK samples had densely clustered ionic groups due to the highly methylbenzylated monomer and thus showed obvious ionic clustering in the membrane. The ion conduction in the QA-PAEK membranes was therefore much greater than in the QA-Radel sample. Increasing the temperature increased the water uptake, swelling ratio, and Br− conductivity of the QA-PAEK samples. As shown in Figures S3 and S4, the water uptake and swelling ratio of the membranes increased gradually with increasing temperature, and at any given temperature, the water uptake and swelling ratio increased with increasing IECg. The Br− conductivities of QA-PAEKs showed an approximate Arrhenius-type temperature dependence promoted by the thermal activation of water motion49 (Figure 5). The apparent activation energy estimated from the slopes of the ln σBr− vs 1/T plots was ∼26 kJ mol−1 and was similar among all the tested membranes (Table 2). The Br− conductivity for QAPAEK-50 increased from 11.8 to 58.9 mS cm−1 as the temperature increased from 20 at 80 °C, the highest among the QA-PAEK samples. Sudre et al.21 reported poly(styrene)based block anion exchange membranes with tethered quaternary ammonium or imidazolium groups. The quaternary ammonium sample SAM (27−21) had an IEC of ∼3.0 mequiv g−1 and a Cl− conductivity of ∼4 mS cm−1 at 25 °C. The imidazolium sample SIM (27−21) had an IEC of ∼2.1 mequiv g−1 and a Cl− conductivity of 1.8 mS cm−1 at 25 °C. While the water uptake values of these linear and non-cross-linked samples were not reported, one can still estimate that the conductivities of the QA-PAEK membranes should be
Figure 6. Water uptake of QA-PAEK samples as a function of relative humidity.
water uptake increased gradually with increasing RH, approaching the water uptake of the samples under fully hydrated conditions. Samples with higher IECs absorbed more water than the samples with lower IECs at any given RH. The Br− conductivity of QA-PAEKs increased greatly with increasing RH (Figure 7). A small increase in RH led to significant increase in Br− conductivity. Therefore, water uptake was critically important for ion conduction. Ye et al.52 reported high conductivity anion exchange membrane with block structures. Their sample, poly(MMA-b-MEBIm-Br-17.3) (IECg = ∼1.4 mequiv g−1 without counteranion), had ∼17 wt % water uptake and ∼1.0 mS cm−1 Br− conductivity at 90% RH 9275
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Figure 7. Br− conductivity of QA-PAEK membranes as a function of relative humidity. Figure 9. Schematic showing (left) low IEC, organized ionic domains, and (right) high IEC, disordered domains.
at 30 °C. Our sample, QA-PAEK-30 (IECg = 2.23 mequiv g−1 with Br− as counteranion), had 16 wt % water uptake and 1.1 mS cm−1 of Br− conductivity at 90 RH at 30 °C. While the IECgs were different, these two samples had similar Br− conductivity at similar water uptake, demonstrating the promising aspects of the QA-PAEK samples, which are still random copolymers, albeit with designed clustered ionic domains through the novel benzylmethylated monomer. To understand the influence of water uptake on the Br− conductivity of these materials, Br− conductivity was plotted versus water uptake as shown in Figure 8. At fixed water uptake,
and high IEC QA-PAEK membranes are schematically displayed in Figure 9. Low IEC membranes with well phase separated ionic domains were observed in the SAXS data in Figure 3. As the IEC increased, the SAXS peak became less distinct and the ionic domains appear to be more disordered. With their high ion content and disorganized domain structure, the high IECg QA-PAEKs needed more water to promote Br− conduction than the low IECg QA-PAEKs. The hydration number, λ (moles of water per mole of quaternary ammonium group), in this series of materials was linked to Br− conductivity (Figure 10). The Br− conductivities
Figure 8. Br− conductivity of QA-PAEK samples as a function of water uptake. Figure 10. Br− conductivity of QA-PAEK samples as a function of hydration number.
−
low IECg QA-PAEK membranes had marginally higher Br conductivity than the samples with high IECg. At fixed Br− conductivity, low IECg QA-PAEK membranes required less water uptake than the high IECg QA-PAEK samples. These interesting results allude to ion conduction in highly hydrated regions of the membranes with organized ionic domains.53 High IECg QA-PAEK samples had more ionic sites to bind water and exhibited more phase mixing than low IECg QAPAEK membranes from the SAXS data in Figure 3. Thus, the high IECg samples had more bound water solvating the ionic groups at the same overall water content compared to the low IECg QA-PAEK samples with organized ionic domains. The data in Figure 8 demonstrate that adding more water to the system does not necessarily increase conductivity unless that water is loosely bound or free and available to participate in ion conduction. The differences in morphology between low IEC
of QA-PAEK-30, QA-PAEK-40, and QA-PAEK-50 samples exhibited a similar dependence on λ, suggesting that the morphology of these samples was fully interconnected and gains in conductivity were only achieved by increased hydration. For QA-PAEK-10 and QA-PAEK-20, the low concentration of quaternary ammonium groups in these materials resulted in reduced Br− conductivity even when they had similar λ to the other QA-PAEK samples. Since ion conduction occurs over the entire length scale of the membrane, on the order of micrometers, the influence of the membrane conducting volume on the ion conductivity is of great interest. The conducting volume should be composed of the volumes of water and ionic groups in the membrane. Stimulated by Kim and Pivovar’s work on calculating the water 9276
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control sample without obvious ion clustering. The Br− conductivity of QA-PAEKs showed an approximate Arrhenius-type temperature dependence under fully hydrated conditions. Under partially hydrated conditions, it was found that high IECg QA-PAEK samples required more water than low IECg QA-PAEK membranes to achieved similar Br− conductivity, perhaps due to better ionic domain morphology in the low IECg membranes. The Br− conductivity of QAPAEK samples increased with increasing conducting volume fraction (Xv) of the samples; however, Xv cannot be simply compared in different QA-PAEK samples because of the changes in ion hydration and water dynamics as the IEC and λ of the samples vary. These results suggested that evaluating different conducting volume character, the connectivity of the ionic domains, and the water motion within the material is critically important in understanding the ion conductivity of AEMs.
volume fraction of the membranes by van der Waals volume increments,54 we included the molar volume of the quaternary ammonium groups with the water volume as the conducting volume using the same method. The conducting volume fraction (Xv) was obtained via the equation Xv =
18λ + VQA ∑ niVi + 18λ
where Vi is the contribution of the ith structural group which appeared ni times per charge. VQA is the molar volume of quaternary ammonium group with Br− as counteranion. The molar volumes of all the structural groups were obtained from ref 55. The relationship between Br− conductivity and Xv for the QA-PAEK membranes is shown in Figure 11. It can be seen
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ASSOCIATED CONTENT
S Supporting Information *
FTIR spectrum of QA-PAEK-30, XPS spectrum of QA-PAEK30, water uptake of QA-PAEK samples as a function of temperature, swelling ratio of QA-PAEK samples as a function of temperature, TGA curves of QA-PAEK membranes, and tensile properties of QA-PAEK and Nafion N212 membranes. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +1 814 867 1847; Fax +1 814 865 2917; e-mail hickner@ matse.psu.edu (M.A.H.).
Figure 11. Br− conductivity of QA-PAEK samples as a function of conducting volume fraction.
Author Contributions
D.C. designed the new monomer, carried out the experiments, and drafted the manuscript. M.A.H. contributed to the analysis of the data and results, developed descriptions of the structure−property relationships of these samples, and helped to refine the manuscript language. All authors have given approval to the final version of the manuscript.
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that the Br conductivity increased with increasing Xv for each QA-PAEK sample, but the higher Xv in the high IECg QAPAEK samples did not guarantee their higher Br− conductivity compared to the low IECg QA-PAEK samples. It seems that the Xv cannot be simply compared in different QA-PAEK samples as factors including water binding and dynamics need to be taken into account. Xv is composed of both the volume occupied by water and the volume occupied by ionic groups, and the ratio of these volumes is different for different samples. For a given water volume, there are free water molecules and bound water molecules whose distributions are dependent on the nature of the samples. Therefore, evaluating both ionic and water conducting volumes, their connectivity, and the dynamics within conducting domains is of critical importance in the understanding of the ion conductivity of AEMs. More work on these topics is needed as design, synthesis, analysis, and deployment of AEMs continue.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the Office of Electricity (OE Delivery & Energy Reliability (OE), U.S. Department of Energy (DOE), under Contract DE-AC05-76RL01830. We also acknowledge the support from the Southern Pennsylvania Ben Franklin Commercialization Institute (Grant #001389002) and the Virginia S. and Philip L. Walker, Jr., Faculty Fellowship to M.A.H. from the College of Earth and Mineral Sciences at The Pennsylvania State University. Dr. Chen thanks Ms. Melanie L. Disabb-Miller for the measurement of RH water uptake and Dr. Nanwen Li for the measurement of SAXS.
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CONCLUSIONS Quaternary ammonium functionalized benzylmethyl containing poly(arylene ether ketone)s (QA-PAEKs) with clustered ionic groups and IECgs ranging from 1.12 to 2.88 mequiv g−1 were synthesized successfully from a newly designed mononer for high performance anion exchange membranes. The densely functionalized quaternary ammonium groups in the samples formed distinct ion clusters in solution-cast membranes, as evidenced by SAXS measurements. A significant increase in the ion conductivity was observed when comparing the QA-PAEK membranes to a quaternary ammonium functionalized Radel
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